Presently, most over-the-road heavy vehicles are fuelled by gasoline or diesel fuel. Because both gasoline and diesel-fuelled internal combustion engines generate a significant amount of pollutants such as oxides of nitrogen (NOx) and particulate matter (PM), engine manufacturers have been searching for best ways to improve their engines to comply with the new government regulatory standards which are becoming progressively more stringent with respect to the allowed levels of pollutants in tailpipe emissions.
For diesel-cycle engines one approach that shows a significant improvement in reducing the levels of pollutants in tailpipe emissions involves substituting a part or all the diesel fuel with cleaner burning gaseous fuels such as natural gas, pure methane, ethane, liquefied petroleum gas, lighter flammable hydrocarbon derivatives, hydrogen, and blends of such fuels. Gaseous fuels are generally defined herein as fuels that are gaseous at atmospheric pressure and zero degrees Celsius. Whereas liquid fuels such as diesel are injected at very high pressures in order to atomize the fuel, gaseous fuels can be injected into an engine's combustion chamber at lower pressures because no extra energy is required for fuel atomization. An advantage of using the diesel-cycle and substituting a gaseous fuel for diesel fuel is this approach can preserve the high efficiency and high torque of the conventional diesel engines, while reducing pollutant levels in tailpipe emissions.
However, some modifications are required to a conventional diesel engine to allow gaseous fuels to be substituted for diesel fuel. In a conventional diesel engine, the heat produced by the mechanical compression of the fuel and air mixture auto-ignites the liquid diesel fuel charge at or near the end of the piston's compression stroke. Other liquid fuels such as dimethyl ether, bio-diesel, and kerosene will also auto-ignite at the temperatures and pressures within the combustion chamber generated by the compression of the charge within the combustion chamber. However, under the same temperature and pressure conditions generated by the compression of the charge within the combustion chamber, gaseous fuels such as natural gas will not reliably auto-ignite. Therefore, in order to reliably burn a gaseous fuel in a conventional compression ignition engine with the same compression ratio as a diesel engine, an igniter is required within the combustion chamber to assist with ignition of the gaseous fuel, such as a hot surface provided by a glow plug, a spark plug, or a fuel injection valve for introducing a fuel that will reliably auto-ignite, acting as a pilot fuel. The pilot fuel can be a small quantity of diesel fuel, whereby the auto-ignition of the diesel fuel triggers the ignition of gaseous fuel.
While gaseous fuels are generally cleaner burning than conventional liquid fuels, tailpipe emissions from gaseous-fuelled engines can be further improved to reduce the levels of NOx by applying a treatment called Selective Catalytic Reduction (“SCR”) to the gases exhausted from the engine. In an SCR converter, ammonia is injected into the exhaust stream upstream of the SCR catalyst as a reduction agent. The ability of ammonia as a reductant to achieve a significant reduction of NOx has been proven for stationary power applications and therefore has been used in diesel-fuelled engines. Other forms of ammonia can be used, such as urea, aqueous, gaseous or liquid ammonia. Using an SCR converter, the SCR catalyst facilitates the reaction between ammonia and NOx to produce water and nitrogen gas.
However, the applicants have found that combining an SCR converter with a gaseous-fuelled engine did not always achieve the same NOx conversion rates. Under some conditions, especially when the engine is idling, it was found that the temperature of the exhaust gas exiting the combustion chamber was significantly lower than the temperatures normally found under higher speed engine operation. To maintain a high NOx conversion rate it was determined that the temperature of the catalytic bed in the SCR converter is preferably above a predetermined temperature which can vary depending upon the composition of the catalyst. Generally, if the temperature of the exhaust gas exiting the combustion chamber is maintained above 200 degrees Celsius, acceptable NOx conversion rates are achieved.
For conventional diesel engines there are many known approaches for increasing the exhaust gas temperature, but there are particular characteristics of gaseous-fuelled engines that prevent the simple transfer of these approaches. For example, some approaches result in unburned fuel being introduced into the exhaust stream, and gaseous fuels, such as natural gas, which consists mostly of light hydrocarbons (methane in particular), do not readily oxidize in the diesel oxidation catalyst of the after-treatment system, especially at lower temperatures, and therefore do not generate heat to be used by the aftertreatment system.
Therefore there are special considerations that need to be taken into account to develop a successful engine system that uses a gaseous fuel and a SCR converter for reducing levels of NOx in the tailpipe emissions.